The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. (Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al., Cell 1992, 70, 419-429; Kunz et al., Cell 1993, 73, 585-596; Garcia-Bustos et al., EMBO J. 1994, 13, 2352-2361).
Many diseases are associated with abnormal cellular responses triggered by the protein kinase-mediated events described above. These diseases include, but are not limited to, autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease, viral diseases, and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
The cyclin-dependent kinase (CDK) complexes are a class of kinases that are targets of interest. These complexes comprise at least a catalytic (the CDK itself) and a regulatory (cyclin) subunit. Some of the more important complexes for cell cycle regulation include cyclin A (CDK1-also known as cdc2, and CDK2), cyclin B1-B3 (CDK1) and cyclin D1-D3 (CDK2, CDK4, CDK5, CDK6), cyclin E (CDK2). Each of these complexes is involved in a particular phase of the cell cycle. Additionally, CDKs 7, 8, and 9 are implicated in the regulation of transcription.
The activity of CDKs is regulated post-translationally, by transitory associations with other proteins, and by alterations of their intracellular localization. Tumor development is closely associated with genetic alteration and deregulation of CDKs and their regulators, suggesting that inhibitors of CDKs may be useful anti-cancer therapeutics. Indeed, early results suggest that transformed and normal cells differ in their requirement for, e.g., cyclin A/CDK2 and that it may be possible to develop novel antineoplastic agents devoid of the general host toxicity observed with conventional cytotoxic and cytostatic drugs. While inhibition of cell cycle-related CDKs is clearly relevant in, e.g., oncology applications, inhibition of RNA polymerase-regulating CDKs may also be highly relevant in cancer indications.
The CDKs have been shown to participate in cell cycle progression and cellular transcription, and loss of growth control is linked to abnormal cell proliferation in disease (see e.g., Malumbres and Barbacid, Nat. Rev. Cancer 2001, 1:222). Increased activity or temporally abnormal activation of cyclin-dependent kinases has been shown to result in the development of human tumors (Sherr C. J., Science 1996, 274: 1672-1677). Indeed, human tumor development is commonly associated with alterations in either the CDK proteins themselves or their regulators (Cordon-Cardo C., Am. J. Pat1/701. 1995; 147: 545-560; Karp J. E. and Broder S., Nat. Med. 1995; 1: 309-320; Hall M. et al., Adv. Cancer Res. 1996; 68: 67-108).
Naturally occurring protein inhibitors of CDKs such as p16 and p27 cause growth inhibition in vitro in lung cancer cell lines (Kamb A., Curr. Top. Microbiol. Immunol. 1998; 227: 139-148).
CDKs 7 and 9 seem to play key roles in transcription initiation and elongation, respectively (see, e.g., Peterlin and Price. Cell 23: 297-305, 2006, Shapiro. J. Clin. Oncol. 24: 1770-83, 2006;) Inhibition of CDK9 has been linked to direct induction of apoptosis in tumor cells of hematopoetic lineages through down-regulation of transcription of antiapoptotic proteins such as Mcl1 (Chao, S.-H. et al. J. Biol. Chem. 2000; 275:28345-28348; Chao, S.-H. et al. J. Biol. Chem. 2001; 276:31793-31799; Lam et. al. Genome Biology 2: 0041.1-11, 2001; Chen et al. Blood 2005; 106:2513; MacCallum et al. Cancer Res. 2005; 65:5399; and Alvi et al. Blood 2005; 105:4484). In solid tumor cells, transcriptional inhibition by downregulation of CDK9 activity synergizes with inhibition of cell cycle CDKs, for example CDK1 and 2, to induce apoptosis (Cai, D.-P., Cancer Res 2006, 66:9270. Inhibition of transcription through CDK9 or CDK7 may have selective non-proliferative effect on the tumor cell types that are dependent on the transcription of mRNAs with short half lives, for example Cyclin D1 in Mantle Cell Lymphoma. Some transcription factors such as Myc and NF-kB selectively recruit CDK9 to their promoters, and tumors dependent on activation of these signaling pathways may be sensitive to CDK9 inhibition.
Small molecule CDK inhibitors may also be used in the treatment of cardiovascular disorders such as restenosis and atherosclerosis and other vascular disorders that are due to aberrant cell proliferation. Vascular smooth muscle proliferation and intimal hyperplasia following balloon angioplasty are inhibited by over-expression of the cyclin-dependent kinase inhibitor protein. Moreover, the purine CDK2 inhibitor CVT-313 (Ki=95 nM) resulted in greater than 80% inhibition of neointima formation in rats.
CDK inhibitors can be used to treat diseases caused by a variety of infectious agents, including fungi, protozoan parasites such as Plasmodium falciparum, and DNA and RNA viruses. For example, cyclin-dependent kinases are required for viral replication following infection by herpes simplex virus (HSV) (Schang L. M. et al., J. Virol. 1998; 72: 5626) and CDK homologs are known to play essential roles in yeast.
Inhibition of CDK9/cyclin T function was recently linked to prevention of HIV replication and the discovery of new CDK biology thus continues to open up new therapeutic indications for CDK inhibitors (Sausville, E. A. Trends Molec. Med. 2002, 8, S32-S37).
CDKs are important in neutrophil-mediated inflammation and CDK inhibitors promote the resolution of inflammation in animal models. (Rossi, A. G. et al, Nature Med. 2006, 12:1056). Thus CDK inhibitors, including CDK9 inhibitors, may act as anti-inflammatory agents.
Selective CDK inhibitors can be used to ameliorate the effects of various autoimmune disorders. The chronic inflammatory disease rheumatoid arthritis is characterized by synovial tissue hyperplasia; inhibition of synovial tissue proliferation should minimize inflammation and prevent joint destruction. In a rat model of arthritis, joint swelling was substantially inhibited by treatment with an adenovirus expressing a CDK inhibitor protein p 16. CDK inhibitors are effective against other disorders of cell proliferation including psoriasis (characterized by keratinocyte hyperproliferation), glomerulonephritis, chronic inflammation, and lupus.
Certain CDK inhibitors are useful as chemoprotective agents through their ability to inhibit cell cycle progression of normal untransformed cells (Chen, et al. J. Natl. Cancer Institute, 2000; 92: 1999-2008). Pre-treatment of a cancer patient with a CDK inhibitor prior to the use of cytotoxic agents can reduce the side effects commonly associated with chemotherapy. Normal proliferating tissues are protected from the cytotoxic effects by the action of the selective CDK inhibitor.
Accordingly, there is a great need to develop inhibitors of protein kinases, such as CDK1, CDK2, CDK3, CDK4, CDK5, CDK6, CDK7, CDK8 and CDK9, as well as combinations thereof.